When operating normally, the core temperature of the human body is regulated somewhere between 98° F. and 100° F. The body uses a variety of heat transfer mechanisms in regulating its core temperature. These heat transfer mechanisms include radiation, conduction, convection and evaporation of water from the body.
When the environmental temperature is below the body temperature, heat is transferred from the body through one or more heat transfer mechanisms of radiation, conduction, convection and/or evaporation of water from the body. This forces the body to conserve heat and increase heat production. When the environmental temperature is above the body temperature, heat is transferred into the body through one or more heat transfer mechanisms of radiation, conduction, and/or convection. In this situation, for example, the body uses the evaporation of perspiration from the skin and the evaporative cooling from exhaled moisture to maintain the core body temperature.
The body takes an active role in regulating the body temperature. The temperature of the body is regulated by neural feedback mechanisms that operate primarily through the hypothalmus. The hypothalmus contains not only the control mechanisms, but also the key temperature sensors. Under control of these mechanisms, sweating begins almost precisely at a skin temperature of 35° C. and increases rapidly as the skin temperature rises above this value. The heat production of the body under these conditions remains almost constant as the skin temperature rises. If the skin temperature drops below 35° C., a variety of responses are initiated to conserve the heat in the body and to increase heat production. These include vasoconstriction to decrease the flow of heat to the skin, cessation of sweating, shivering to increase heat production by the muscles and the secretion of norepinephrine, epinephrine, and thyroxine to increase heat production.
Different regions of the body have different abilities to transfer heat. This is based in part on both the surface area of the body region and the relative vascularization of the body region. For example, up to 40% of the body's heat is lost from the head. This is due to the large blood supply to the head, the extra surface area of the head, and from water evaporating from the nose and mouth. Other areas where there is a large muscle mass also coupled with a large surface area also can lose a significant amount of heat.
Clothing is used to help maintain the body core temperature. For example, additional layers of clothing are worn to help maintain body temperature in cold environments. Conversely, fewer layers of clothing are worn to help maintain body temperature in warmer environments. Regulating body temperatures due to a change in a person's activity level in these conditions is usually as simple as adding or subtracting additional layers of clothing.
In some situations, the person is unable to remove or add additional layers of clothing when their activity level changes. For example, people working in very hostile environments are not able to remove or add to their protective clothing and/or suits if they become too hot or cold (e.g., astronauts, fighter pilots, firemen, divers, etc.). In some situations it is also possible that a person can be too warm in one region of their body, while another region is too cold.
Garments for controlling the core body temperature have been suggested. These garments typically have a system for circulating temperature controlling fluid into and out of the garments, e.g., via tubes positioned in the garment. The temperature controlling fluid is generally circulated through the entire garment, regardless of whether the body area needs to be heated or cooled. In doing so, the system wastes energy. For example, energy is wasted in pumping temperature controlling fluid that may not be needed in a particular area of the suit. In addition, the efficiency of the system is reduced. Reduced efficiency results when areas that may require more heating or cooling fail to receive sufficient temperature controlling fluid to control the body temperature because a portion of the finite amount of temperature controlling fluid may be circulating in areas that do not require heating or cooling at that time. Furthermore, the garments used to control temperature of the body are generally heavy and cannot be worn for long periods of time.